Utilization of DRI in the steelmaking industry is expanding and the number of so-called mini-mills, which are steel plants of relatively small and medium production capacity, is increasing. These mini-mills comprise one or more direct reduction plants, wherein DRI is produced from iron ores, and electric arc furnaces (EAF) for melting said DRI and producing liquid iron and steel.
The economic and operational advantages of modern steelmaking mini-mills have been recognized in the prior art and some methods and apparatus have been proposed for rendering the mini-mills facilities more efficient with lower investment and operational costs.
See for example U.S. Pat. No. 6,478,841 to Faccone et al, which teaches a method of making steel in an integrated mini-mill wherein the DRI produced in a reduction reactor is transported at high temperatures to the EAF by means of an inclined rotary kiln. The DRI is discharged from the reduction reactor by a screw feeder and then is transported through the gravity-aided rotary kiln (which is pressurized with part of the top gas effluent from said reduction reactor in order to protect the hot DRI from re-oxidation). Faccone discloses the desirability of developing hot DRI handling systems capable of delivering hot DRI to the EAF with a minimum loss of metallization, i.e. the proportion of metallic iron content relative to the total iron content, which are easy to operate and having a low capital investment cost.
U.S. Pat. No. 5,296,015 to Becerra-Novoa et al (and assigned to an affiliated company of applicants' assignee) teaches a method of pneumatically transporting hot DRI produced in a direct reduction reactor capable of delivering such hot DRI with greater flexibility, especially to remote points of use. This patent generally discloses the method of pneumatically transporting hot DRI, but it is mute concerning the details of the system for charging DRI to the EAF and of the economic impact of minimizing the height, and therefore the cost, of the supporting structures of the DRI reactor, of the DRI charging bins and of a DRI cooler for discharging cold DRI for its safe handling, storage or transport at temperatures below about 100° C. This patent also does not teach or suggest any solution for the practical match of the continuous hot DRI production and the batch operation of the hot DRI melting furnace.
U.S. Pat. No. 5,445,363 also to Becerra-Novoa et al is a continuation-in-part of the foregoing U.S. Pat. No. 5,296,015 and adds a disclosure of a method and apparatus for producing iron and steel which addresses with some particularity the problem of minimizing the height of the reduction reactor supporting structure, such as when the hot DRI is used to produce briquettes of DRI. Becerra-Novoa et al here teach that the height of the support tower 142 (in FIG. 7) may be decreased by utilizing a pneumatic transport system for conveying the iron ore charged to and/or from the reduction reactor but does not address in any way the benefits that may be obtained by pneumatically transporting hot DRI, especially in an mini-mill plant having an electric-arc furnace where there are a number of operational and lay-out constraints and where there is the need of minimizing the operational and investment costs of the mini-mill plant.
The structure of the pneumatic transport system disclosed in the two Becerra-Novoa et al patents is elaborated upon with an added detail in a paper entitled “Super-Integration: Use of Hot DRI at New Hylsa CSP Mill”, presented at the Nov. 20-22, 1996 Gorham/Intertech Mini-Mills of the Future Conference in Charlotte, N.C. This paper describes a proposed scale up of an existing pilot plant (that had been used to prove the concepts of the Becerra-Novoa et al patents). In comparing FIGS. 2 & 3 of the Becerra-Novoa et al patents with FIG. 2 of the paper, it will be seen that the patents show a carrier gas disengagement bin 54 together with a separate depressurizing lockhopper 130 (for feeding the EAF 48). In contrast, FIG. 2 of the paper shows a parallel pair of “EAF feeding bins”; where the disengagement & depressurizing functions are combined into each of such pair of bins, and the pair of such bins alternate with each other to give a continuous feed of DRI charge from the reduction reactor to the EAF (with one pressurized and filling with DRI, while the other is depressurized and discharging DRI to the EAF).
The structure of the paper's FIG. 2, as actually constructed in a mini-mill 1998, is shown diagrammatically on page 6 of the publication entitled HYL Report-The Direct Reduction Quarterly, Summer 2000, Vol. XIV, No. 2, published by HYL, Monterrey, Mexico. See the parallel bins feeding the “DC Furnace No. 1.” Similarly, in the same mini-mill, the structure of the two Becerra-Novoa et al patents was also incorporated and expanded upon for commercial scale up. As illustrated and actually constructed, the upper three series of bins feeding the “DC Furnace No. 2” functioned respectively as 1) a simple disengagement bin 54, 2) a depressurizing bin 103, and 3) an atmospheric holding bin of sufficient capacity to have the charge to the EAF be 100% DRI. Note that each small bin immediately above each EAF is equivalent to bin 52 in appended FIG. 1 of this present application. Also note that the other two of the three parallel bins feeding “DC Furnace No. 2” are also atmospheric bins, two of which receive DRI from sources separately or indirectly from the reduction reactor (such as from the “external cooler”). This plant has been in operation now for a decade. This publication is currently available online at http://www.hylsamex.com/hyl/reportes/2000/summer.pdf.
U.S. Pat. No. 6,214,086 to Montague et al teaches a system using gravity to transport hot DRI material from a reduction reactor to an EAF and/or to a cooling vessel. This patent, at the bottom of column 1 and in the middle paragraphs of column 3, specifically teaches way from using pneumatic transport of the DRI, yet is a good demonstration of the limitations and drawbacks of a gravity feed system (limited travel distances and costly structural support of the large reactor 10 at great heights in order to be sufficiently above the EAF and the cooler, so as to achieve the necessary gravity feed angles thereto).
For further background, see U.S. Pat. No. 4,528,030 to Martinez-Vera et al, which shows a direct reduction plant which can be used in conjunction with the present invention.
DRI is a solid granular material which is produced by reaction of iron ores (mainly iron oxides) in solid phase with a reducing gas at a high temperature on the order of 900° C. to 1100° C. in a reduction reactor with or without a DRI cooling zone. DRI is then melted, preferably in an electric arc furnace, to produce molten iron and transformed into liquid steel. Direct reduction plants typically comprise a continuous moving bed reactor discharging hot or cold DRI. The term “cold DRI” is applied to DRI discharged at temperatures preferably below about 100° C., and the term “hot DRI” is applied to DRI discharged at temperature typically above about 400° C., e.g. to DRI not cooled down in the reduction reactor.
DRI, melted-down in electric arc furnaces, is usually mixed with scrap in selected proportions according to the economic cost of the charge materials and the attainable quality of the final steel products. The technology of design and operation of electric arc furnaces has evolved considerably. These melting furnaces utilize both electrical and chemical energy for decreasing the tap-to-tap time thus increasing the productivity of the furnace. In this respect, DRI containing a high proportion of combined carbon (above about 3% of Fe3C) is significantly beneficial, because this carbon chemically combines with oxygen injected into the furnace producing heat and a foamy slag resulting also a number of other advantages.
Since hot DRI reacts with oxygen and moisture, if exposed at high temperatures to the ambient air, it is necessary to provide a shield of inert atmosphere while handling hot DRI until it reaches the EAF. There are several proposals in the industry for transferring hot DRI to a melting furnace. One in more recent use is by means of a pneumatic transport system utilizing an inert gas or a reducing gas as the carrier gas. Another way has been by means of moving the hot DRI on rails or by crane in individual refractory-lined closed transport vessels. Also, gravity feed has been traditionally used, but is severely constrained to short transport distances by the required angle of flow and by the expense of supporting the massive reactors at substantial heights to be above the structures being fed and especially to achieve such flow over any meaningful distance that might be needed to reach such structures.
The need exists for a method and apparatus for the design, construction and operation of an efficiently laid out mini-mill plant based on the respective needs of DRI production and of the DRI melting furnace (and not on the needs of the DRI transport system). The present invention provides such method and apparatus, while providing also a number of other advantages over the above-referenced prior-art systems.
Documents cited in this text (including the foregoing patents), and all documents cited or referenced in the documents cited in this text, are incorporated herein by reference. Documents incorporated by reference into this text or any teachings therein may be used in the practice of this invention.